1. Field of the Invention
The present invention relates to a multi-layered organic light-emitting diodes.
2. Description of the Related Art
Recently, with a solution process applicable to not only a polymer organic light-emitting diodes but also a low molecular weight organic light-emitting diodes, many attempts are being made to apply a printing method to the solution process. The solution process includes an inkjet printing, a roll-to-roll printing, a screen printing, a spray coating and a dip coating and the like.
However, an anode and a cathode are still manufactured in vacuum by sputtering or evaporation. Since the aforesaid process requires a high-degree vacuum technology as well as very expensive process equipments, it is required that the number of vacuum deposition processes should be reduced, considering productivity and manufacturing cost.
An electron injection layer is formed by depositing an ultrathin film which is made of LiF, CsF, NaF and Cs2CO3 and has a thickness of about 1 nm. Otherwise, the electron injection layer is formed of a layer which is made of Ca, Li, Ba, Cs and Mg, etc., and has a thickness of about 20 nm. When a cathode is additionally deposited, the above layer is very vulnerable to oxygen and moisture in the outside air, so that the life span of the element is reduced. Moreover, these materials are not easy to handle during processing.
Since an ultrathin electron injection layer of the most used electron injection layer/cathode structure like LiF/Al uses a thin film of 0.5 to 2 nm, very important is a surface condition of coating film of an under layer on which the ultrathin electron injection layer is stacked. Therefore, since it is not easy to obtain a sufficient coating performance through the solution process such as a roll-to-roll coating, an inkjet printing, a screen printing, a spray coating and a dip coating, it is very difficult to apply the ultrathin electron injection layer to the solution process.
Meanwhile, in most conventional low molecular weight light-emitting diodes/polymer light-emitting diodes, the organic light-emitting diodes is manufactured by depositing a cathode in a high-degree vacuum through the use of an alkali metal or alkaline earth metal which has a low work function. When a metal having a high work function is used as a cathode, there is a problem that the performance of the organic light-emitting diodes is greatly reduced. When an alkali metal or alkaline earth metal which has a low work function is used as a cathode, such a cathode is vulnerable to oxygen and moisture in the air. Accordingly, it has been difficult to handle the cathode and a high-degree vacuum had to be maintained during the process.
As a result, when a cathode made of a metal having a high work function like Ag, Au and Al is used, an attempt is recently made to prevent the performance of the organic light-emitting diodes by mixing ammonium ion with a light emitting material or by coating an emissive layer with a polymer material having an ammonium ion group.
However, a polar solvent must be used for the polymer material having an ion group or an electrolytic ion complex having an ion group. Therefore, when the polar solvent is coated on an organic emissive layer, a polar polymer material having an ion group is agglomerated, and when the polar solvent is coated on a hydrophobic emissive layer, a aggregated coating degradation like stain occurs, so that the hydrophilic polymer material or an electrolytic ion complex, which has an ion group, is difficult to be applied to mass production due to their coating problems. Accordingly, in most cases, it is required that a material having an ion group be doped in or added to the emissive layer, and ions freely float within the emissive layer, which has a bad influence on the emission performance of the device.